This invention relates to data communication systems and, more particularly, to continuous phase shift modulation systems.
Continuous phase shift modulation (sometimes referred to as minimum frequency shift keying) is a known and commonly used technique for modulating digital data onto a radio frequency carrier wave for transmission and subsequent reception. The baseband representation of a continuous phase shift modulation signal is comprised of an in-phase and a quadrature component, each of which is composed of a continuous sequence of half-cosine pulses of varying polarities. The quadrature pulses are time offset from the in-phase pulses by 90.degree., so that the resulting vector sum amplitude is a constant. An advantageous result of this technique is that the modulation affects only the phase of the carrier signal and not the amplitude. This allows for the use of highly non-linear radio frequency transmitter power amplifiers, thereby reducing cost and size and increasing efficiency.
In order to facilitate the explanation that follows, the logical relationship between incoming data bits and the in-phase and quadrature pulses has been simplified from that usually employed in practice. This simplification is only for descriptive purposes and is not intended to limit the scope of this invention.
In a continuous phase shift modulation system, the polarity of each in-phase pulse is determined by successive even incoming data bits and the polarity of each quadrature pulse is determined by successive odd incoming data bits. Accordingly, when successive alternate incoming data bits have the same value, adjacent half-cosine pulses in a component will have the same polarity. This results in a sharp discontinuity at the juncture between these two adjacent pulses. The discontinuity is rich in high frequency components, resulting in spectrum spreading, which is an undesirable attribute. It is known that filtering can improve spectral roll off, but filtering introduces amplitude modulation which then requires hard limiting as a final operation to re-establish the constant amplitude characteristic of the original continuous phase shift modulation signal. However, the use of limiters introduces excessive complexity and cost, and even then it is difficult to achieve adequate limiter performance.
It is therefore an object of the present invention to provide improved spectrum control in a continuous phase shift modulation system.
It is another object of this invention to provide such improved spectrum control while strictly maintaining the constant amplitude characteristic of the signal, thereby eliminating the need for hard limiting.